The typical small solid state gamma ray detector is a device which employs a single crystal of a material, such as cadmium telluride or silicon, to detect the presence of gamma rays.
For the applications contemplated for these small solid state gamma ray detectors, such as a pocket dosimeter for use on the nuclear battlefield, it is desirable to read dose or dose rate in units that correspond to the biological effect of the radiation, that is, in rads (tissue). One rad (tissue) is the radiation dose equal to an energy absorption of 100 ergs per gram of tissue. Similarly, one rad (detector material) is the radiation dose equal to an energy absorption of 100 ergs per gram of detector material. Another unit, the roentgen (R), is defined as the amount of radiation that produces one esu of charge in one cc of dry air at standard temperature and pressure. It can be expressed as that amount of radiation which produces 2.58.times.10.sup.-4 coulombs of charge per kilogram of air. Since the fraction of the total energy that goes to producing free charge is constant, one roentgen (1 R) of radiation corresponds to 0.869 rads (air).
When gamma rays are absorbed by matter, the gamma ray energy is typically transferred to electrons which, in turn, transfer the energy to the rest of the mass, producing ionization, bond dissolution and eventual heating of the medium. In the case of a solid state semiconductor detector, the electron energy creates electron hole pairs in the semiconductor material which can carry signal current. Since the amount of charge produced is directly proportional to the energy deposited by the energetic electrons, the output of the ideal detector will be directly proportional to the energy deposited per unit mass of detector material. This output is, of course, measured in rads (detector material) rather than in rads (tissue).
To date, no pulse counting solid state detector has been developed which has the same density and atomic numbers as tissue. The one class of tissue equivalent solid state detectors that has been successful is the recently developed radiochromic dosimeters which have a very good match to tissue in terms of their atomic number, and thus give a response directly proportional to rads (tissue). One such device is disclosed in U.S. Pat. No. 4,489,240, issued to Kronenberg, et al. on Dec. 18, 1984. The response of these devices is, however, proportional to the integrated dose and, as such, their ability to act as dose rate meters is limited by their dynamic range and the maximum total dose before replacement of the sensitive element is required.
Another conceivable development is the use of industrial diamonds as very low atomic number semiconductor detectors, but the size, electronic properties and cost of this material makes this seem a farfetched approach. There have also recently been many developments exploring the semiconducting properties of polyacetylenes and other related organic polymers, but to date none of these has achieved the high resistivities needed for the direct detection of ionizing radiation.
Thus for any known semiconductor with sufficiently good electronic transport properties to act as a pulse counting radiation detector, there is a significant deviation in atomic numbers from those of tissue, and thus a correction must be made to convert the detector signal into rads (tissue). In particular, for a silicon detector of the type tentatively chosen for the pocket dosimeter, at least three effects must be taken into account. The first is that the energy dependence of the gamma ray absorption for silicon is quite different from that of tissue, since the higher atomic number of silicon significantly enhances the fraction of low energy gamma rays which will be absorbed.
The second factor is that the electrons which receive the energy from the gamma rays can travel much further in tissue than in silicon. This will change the depth dependence of the energy response of the silicon with respect to tissue.
A third factor which must be considered arises from the fact that the detector will have a finite, and usually small, size. This means that some gamma ray energy absorbed by material external to the detector will result in electrons entering the detector and producing signal. Similarly, some gamma ray energy absorbed in the detector will produce electrons which will leave the detector and therefore not produce the expected detector signal.
For a different detector material, such as cadmium telluride, these same factors acquire a different emphasis. Because of its higher atomic number, CdTe has a much more exaggerated response at low energies, and thus presents a more difficult case for dose compensaton. Since CdTe detectors are typically much thicker slab type devices than silicon diodes, the effects of electrons leaving the sensitive volume set in only at much higher energies, and could probably be neglected. The main advantage of CdTe in this context is its much greater sensitivity, which would allow measurement of dose rates down to background levels in reasonable time periods.
Thus in order for a detector to read directly in rads (detector material), it should be surrounded on all sides by a layer of the same material as the detector itself. Ideally the dimensions of the whole assembly should be much less than the range of the gamma rays, and the surrounding layer should be thicker than the range of the electrons emitted upon gamma ray absorption. The first condition assures that the dosimeter will respond to the full ambient flux, while the second guarantees that all the energy deposition mechanisms originate within the same type of material. In one embodiment of the present invention this requirement for cladding the detector with detector material can be relaxed. All that is necessary is that the cladding material be the same on all sides of the detector.
One great obstacle in making a small, tissue equivalent, solid state detector is in determining the relationship between the actual detector signal and the desired signal in terms of rads (tissue).
Note first that it is better to integrate the total charge signal produced by each of the detector pulses rather than simply to count the number of detector pulses. This is because it is the sum of the energy deposited by the gamma rays which is important rather than the number of rays which are absorbed. Integrating the charge converts the detector output to a signal proportional to rads (detector material).
If we examine the flux of incident gamma rays needed to produce an exposure rate of 1 R/hr, as a function of energy, we would see that the number of rays which produce this exposure decreases almost linearly with energy, in the energy range from 100 keV to 1 MeV. This indicates that the energy deposited in tissue by the gamma ray stream increases linearly with the incident energy. Thus, if it were possible to construct a tissue equivalent, solid state detector, an integration of the charge produced per unit time could produce an output directly proportional to rads (tissue).
The detector tentatively selected for the pocket dosimeter is made from silicon and it will therefore be necessary to modify its signal by various means to take into account the fact that both its density and average atomic number (Z) are significantly higher than those of tissue. Traditional approaches to tailoring the energy response of gamma ray detection instruments have relied on the use of absorbing foils which are used to attenuate the lower energy radiation and thus to reduce the over-response of the detector in this range. These approaches have always met with significant difficulties because available shielding materials cannot be combined to produce to sufficient accuracy the necessary conversion factor, especially when considering different angles of penetration. The other disadvantage of this approach is that the presence of any such shielding inevitably reduces the overall sensitivity. It is for these reasons that I chose not to shield the detector, but rather to use a microprocessor to compensate the response based on the information that is already available in a high resolution solid state detector.
However, it is important to first show that it is possible, at least in principle, to achieve a flat energy response from a non tissue equivalent detector. The argument is clearly stated: the pulse height spectrum produced by an ideal, non tissue equivalent, calibrated, solid state detector provides exact information on the magnitude of the gamma ray flux at each energy. This information is sufficient to calculate the exact dose in rads (tissue).
It can be shown that, for most cases of importance in solid state detectors, there is sufficient dose information available from the pulse height spectrum of a non ideal, non tissue equivalent, calibrated, solid state detector to yield the tissue dose. That is, even if the response of the detector is not flat, but is in fact quite nonlinear, dose and dose rates in terms of tissue dose can be accurately determined. This nonlinearity arises principally because of the difference in the energy dependent gamma ray stopping power of the detector compared to tissue.
In order to develop a device to compensate for the differences in response for the silicon and the tissue, we consider in detail the relationship between the output of the detector and series of incident monoenergetic gamma rays. This output can be displayed as a pulse height spectrum and clearly demonstrates that the detector response to an individual high energy photon is a probability distribution rather than a single value pulse height. Thus for a single event, it is not possible to learn much about the incident gamma ray energy, but by using a statistical approach for a collection of gamma rays, the dose information can be extracted.
While it is possible to analyze the raw output of a detector to obtain information on the incident gamma ray spectrum, and thus, on the dose and dose rate in rads (tissue), it is impossible at present to do an exact analysis with equipment which can be accommodated within the size, weight, and power constraints presented by the tactical nuclear battlefield.
In particular, while the detector electronics may be miniaturized, the detector itself must be large in physical dimensions and total mass in order to totally absorb the expected gamma radiation and yield an accurate picture of the incident photon spectrum.
A practical device must be smaller than this, and since it is small, it cannot absorb all the incident radiation and fully sample the gamma ray flux.
Those concerned with the development of small solid-state gamma ray dosimeters have long recognized the need to address these and other related problems and to develop such a detector which directly measures radiation absorption in terms of the effect on human tissue. The present invention meets this need.